1. Field of the Invention
The present invention relates to a battery pack including a primary cell and a secondary cell.
2. Description of the Prior Art
A battery pack which can be charged is normally constituted by one type of secondary cell. The battery built in the battery pack is a primary cell or a secondary cell. Here, the primary cell may be a manganese cell, alkali cell, lithium cell, air cell, and the like. The secondary cell may be nickel-cadmium cell, nickel hydrogen cell, lithium-ion cell, lithium polymer cell, and the like.
For example, a drive battery pack for a portable telephone is normally constituted by one type of secondary cell which is a single lithium-ion cell. A battery pack serving as a drive source of a portable personal computer is constituted by one type of secondary cell which is one to nine cells. Moreover a battery pack serving as a drive power source for a portable video camera is constituted by a secondary cell which is one to nine cells.
In the aforementioned battery pack having a conventional secondary cell, when the battery capacity has become small, the battery pack should be charged using a charger and a domestic power source (AC100V) for several hours.
Accordingly, when a user uses an portable electronic apparatus outside home and no battery capacity is left, the user cannot use the portable electronic apparatus any more.
On the other hand, there has been suggested a battery pack which can drive a portable electronic apparatus using a primary cell instead of the secondary cell when the battery capacity become none. However, such a battery pack has a problem that the secondary cell cannot be charged while the primary cell is mounted.
It is therefore an object of the present invention to provide a battery pack which can mount a primary cell to increase the battery capacity and charge a secondary cell while the primary cell is mounted.
The battery pack according to the present invention achieves the aforementioned object by including: a primary cell; a secondary cell; an I/O terminal through which discharge voltage is output or charge voltage is input; select means for selecting a primary terminal connected to the primary cell or a secondary terminal connected to the secondary cell, so as to connect the selected terminal to the I/O terminal; comparison means for comparing the voltage of the primary cell and the secondary cell and, during discharge, controlling the select means so as to select the primary or the secondary terminal connected to a higher voltage, and during charge from an external power source, controlling the select means so as to select the secondary terminal; an inverse current preventing means for connecting the I/O terminal to the secondary cell, so that current flows from the I/O terminal to the secondary cell and not vice versa.